The following publications are representative of the most relevant prior art known to Applicants at the time of filing of this application:
______________________________________ UNITED STATES PATENTS 3,959,557 May 25, 1976 A. R. Berry 4,134,759 January 16, 1979 S. Yajima et al. 4,252,062 February 24, 1981 N. Nishida 4,505,720 March 19, 1985 T. Gabor et al. 4,708,542 November 24, 1987 D. J. Emmanuelli FOREIGN PATENT APPLICATION DE 3608734 January 2, 1987 Feldmuhle AG ______________________________________
Titanium nitride coatings have been placed on various materials to provide wear resistance or corrosion resistance or other similar properties. Generally these coatings have been produced on metal articles by chemical vapor deposition (CVD) or similar processes. For instance, U.S. Pat. No. 3,959,557 discloses the CVD deposition of TiN to produce wear-resistant, nonabrasive surfaces on plain carbon steel articles, and U.S. Pat. No. 4,252,862 discloses depositing TiN on external ornamental metal parts such as wrist watch cases, bands or bezels to obtain a gold-colored surface having corrosion and wear resistance properties. Also, U.S. Pat. No. 4,708,542 discloses that a TiN coated metal tap has superior life to an uncoated tap.
In addition, titanium nitride coatings have been placed on ceramics by CVD processes. U.S. Pat. No. 4,134,759 discloses that metal or ceramic coated continuous silicon carbon fibers which contain a substantial amount of free carbon have improved wettability with aluminum and aluminum alloys when coated with various compounds, including, among others, titanium nitride. No layer intermediate the silicon carbide continuous fibers and the titanium nitride is suggested.
U.S. Pat. No. 4,505,720 discloses the CVD deposition of titanium nitride, among others, directly upon granules of silicon carbide. In this patent, there is no low diffusivity layer between the silicon carbide core and the titanium nitride coating. The granules are quite large, thereby making CVD suitable for performing the coating process. The CVD process, when performed on very fine silicon carbide materials, has been found not to work to produce a complete titanium nitride coating. The process of the present invention permits the titanium nitride coating of such very fine materials.
German Appln DE 3608734 discloses ceramic cutting tools having, among others, titanium carbonitride coatings. The tools are prepared from sintered ceramic bodies which are coated with an intermediate bonding layer of silica which is deposited by either CVD or plasma vapor deposition (PVD). Then the silica layer is CVD or PVD-coated with a cutting layer of alumina, zirconia, hafnia, their mixture, or a titanium carbonitride. The silica interlayer is stated to improve the adhesion of the hard outer coating to the sintered ceramic core. The present invention is not directed to placing coatings on sintered ceramic bodies, but rather on individual particles, fibers or whiskers of silicon carbide, and avoids using CVD to deposit the titanium nitride coating.
Nitriding of titanium to convert the metal to titanium nitride is a well-known procedure. Normally, it readily occurs by heating the titanium metal to an elevated temperature in a nonoxidizing nitrogen-containing environment for a sufficient time to cause the nitridation. However, when titanium metal was coated onto a fine silicon carbide powder and then placed into a nitrogen environment at temperatures up to 1500.degree. C. for up to four hours, the surface did not turn brown or gold as would have happened if titanium nitride had formed. X-ray diffraction confirmed that little or no titanium nitride formation occurred.
Since titanium nitride has substantially increased wettability and/or less solubility for certain metals and metal alloys, particularly aluminum and its alloys, than do both the uncoated silicon carbide materials and silicon carbide coated with many other materials, it is an object of the present invention to produce a titanium nitride coating on fine silicon carbide materials, particularly those materials which are not readily uniformly coated by a chemical vapor deposition process, to enhance their compatibility with such metals and metal alloys.